1. Field of the Invention
This invention relates to a trace continuity tester for bare printed circuit boards, particularly unseparated miniature circuit boards having trace separation of less than about 0.5 millimeter. Specifically, the invention relates to a probe which is specific to the circuit boards to be tested and a method for constructing such a probe.
The construction or printed circuit boards according to conventional photoresist and electroplating processes wherein a conductive plating of copper, nickel, silver or gold is registered on a glass impregnated epoxy substrate is subject to manufacturing effects. Typical manufacturing effects are undesired short circuits between adjacent traces and broken traces giving rise to open circuits. Considerable time and effort is required to locate and eliminate circuits having trace defects.
The two principal methods for locating circuits with trace defects are visual inspection and electrical continuity inspection. In visual inspection, a worker examines each trace of each circuit visually, and possibly under a microscope or lupe, if the circuit is miniature (that is, if the spacing amoung traces is less than can be readily resolved with the unaided eye). Visual inspection is tedious and frequently unreliable, since circuit defects can easily be overlooked.
In the second method most often used, electrical circuitry is tested by mechanically contacting selected test points of the unit under test and electrically transmitting continuity information to an analyzer to determine if there are errors indicative of trace defects. In the past, elevated probe contacts mounted to a general purpose support frame have been employed. One type of probe contact array is the so-called "bed of nails" in which rigid pins are mounted to a board at selected locations corresponding to test points on the circuit board to be tested. Another type of probe contact is the spring-loaded probe contact wherein each terminal comprises a barrel, a spring and a contact plunger mounted to a socket or other support in an array similar to a "bed of nails". The "bed of nails" requires careful plane alignment to assure proper electromechanical contact between the individual pins and the test points. Spring-loaded probe contacts partially overcome this deficiency by compensating for irregularities in the surface of the device to be tested.
There is nevertheless a physical limitation on the minimum size of the contact head and the minimum center to center spacing between contact heads. Typically, a probe contact can be spaced no closer than about 0.7 millimeter center to center. Probes having smaller contacts for lesser diameter spacing are extremely fragile and unreliable. Accordingly, what is needed is a probe capable of use with miniature printed circuit boards.
2. Brief Description of the Prior Art
The following companies are representative of those which provide trace analyzer devices employing the spring-loaded or "bed of nails" type contact probes: Trace Instruments of Canoga Park, Calif.; Program Data, Inc. of Irvine, Calif.; Pylon Company, Inc. of Attleboro, Mass.; Fault Finders, Inc. of Irvine Calif.; Everett/Charles, Inc. of Pomona, Calif.; and ATEC Assembly and Test Equipment Corporation of Pomona, Calif.